This invention relates to a strong magnetic field generator using protective coils together with superconductive coils.
Recently, in apparatus requiring strong magnetic fields such as nuclear fusion apparatus and particle accelerators superconductive coils are used in the strong magnetic field generator.
FIG. 1 shows an example of the strong magnetic field generator used for a nuclear fusion apparatus. An annular or torus vacuum vessel 1, in which plasma is trapped, is provided with eight superconductive coils 2 to 9 wound at a uniform interval on its outer torus surface. The superconductive coil 2 is connected to a power source circuit 10 and energized therefrom. The other coils 3 to 9 are also connected to the power source circuit 10 to be energized thereby, but the connection involved is not shown here.
The power source circuit 10 has a construction, for instance, as shown in FIG. 2. Reference numeral 11 designates a power supply which is a combination of an AC power source and a thyristor rectifier. DC current from the power supply 11 is supplied through a DC circuit breaker 12 to the superconductive coils 2 to 9. The superconductive coils 2 to 9 are connected in parallel with a discharge resistor 13. They are each formed by winding a superconductor 15 as shown in FIG. 3, consisting of a central superconductive wire 16 and an outer conductor 17 surrounding the central superconductive wire 16, into the form of a coil. In the superconduction state, current flows through the superconductive wire 16, but when ordinary conduction of the superconductive wire 16 is brought about some of the current gets out from the superconductive wire 16 and flows through the outer conductor 17. The superconductive coils 2 to 9 are held in a cooling medium (not shown) and cooled thereby.
There is a certain maximum current or threshold current that can be caused to flow through the superconductive wire 16, and this threshold current is a function of the temperature of the superconductive wire 16 and external magnetic field. Thus, when ordinary conduction of part of the superconductive wire 16 is brought about due to such cause as a temperature rise or a change of the magnetic field, some of the current that has previously been flowing through the superconductive wire 16 migrates therefrom into the outer conductor 17. This phenomenon is called quenching. The occurrence of quenching in the superconductive coils 2 to 9 leads to a grave accident in the nuclear fusion apparatus that may be caused by Joule heat generated in the outer conductor 17, thermal expansion or evaporation of cooling medium, rupture or burning of the superconductive coils 2 to 9 or other causes unless appropriate protection is provided.
A prior art protection system which is adopted for providing protection in case of occurrence of quenching in a superconductive coil uses a discharge resistor 13 as shown in FIG. 2. With this system, upon occurrence of quenching in one of the superconductive coils 2 to 9, for instance in coil 2, the DC circuit breaker 12 is immediately opened to disconnect the power supply 11 and coil 2 from each other and cause the remaining electric energy or current in the coil 2 to be discharged through the resistor 13, thus removing the quenching of the coil 2. This prior art protective system is very simple and economical in case if the strong magnetic field generator comprises only a single superconductive coil. However, when it is used for an apparatus comprising a number of superconductive coils as shown in FIG. 1, it presents the following various problems.
(1) In case when quenching is brought about in, for instance, the superconductive coil 2 in the system of FIG. 1 and removed by quickly reducing the current in the superconductive coil 2 with the protection system shown in FIG. 2, the currents in the other superconductive coils 3 to 9 are quickly increased due to the law of constant magnetic flux, that is, with mutual induction among the superconductive coils and sometimes exceed the threshold value. In other words, quenching produced in one of the superconductive coils is propagated progressively to the other coils, thus leading to a danger of a fault of the whole system.
(2) When quenching results in one of the superconductive coils in a system having a number of superconductive coils as shown in FIG. 1 and the current in the coil with quenching therein is reduced, an unbalance of the magnetic field is produced to exert forces tending to turn down the system to the superconductive coils. Therefore, grave rupture of the apparatus is liable unless the individual superconductive coils are supported with very great mechanical forces.
(3) When quenching occurs in one of the superconductive coils and is removed by reducing current in the coil with quenching therein, an unbalance of the magnetic field is produced, which is very inconvenient from the standpoint of trapping plasma.